Due to the increased legalization of marijuana in the US, profits for Mexican-produced marijuana have been falling. It's definitely true that Mexican marijuana farmers might turn to heroin if they find the profits to be higher. No matter where you turn, there will always be a market for illicit drugs.
Due to the decline in profitability for marijuana, Mexican marijuana farmers are bound to turn to the heroin business. One major reason that can prompt this switch is the recent laws in the United States legalizing marijuana. Marijuana legalization is causing consumers to acquire higher-quality marijuana from producers in the United States as opposed to producers from Mexico who sell lower-quality marijuana.
If someone already owns or farms a marijuana field, I would assume they are making plenty enough money as is. Going through the process of switching recreational drug would seem ridiculously difficult, seeing as their current product had much time invested into it. And honestly, Marijuana is not a gateway drug.
Most of these people that farm marihuana are peaceful folk. Contrary to popular belief marihuana is NOT a gateway drug. Just because these Mexican farmers are growing marihuana does not mean they are moving on to harder stuff. That is absolutely ridiculous and thus making this a pointless question. Let these people just do their thing.